11,875 research outputs found
The (Double) White Dwarf Binary SDSS 1257+5428
SDSS 1257+5428 is a white dwarf in a close orbit with a companion that has
been suggested to be a neutron star. If so, it hosts the closest known neutron
star, and its existence implies a great abundance of similar systems and a rate
of white-dwarf neutron-star mergers similar to that of the type Ia supernova
rate. Here, we present high signal-to-noise spectra of SDSS 1257+5428, which
confirm an independent finding that the system is in fact composed of two white
dwarfs, one relatively cool and with low mass, and the other hotter and more
massive. With this, the demographics and merger rate are no longer puzzling
(various factors combine to lower the latter by more than two orders of
magnitude). We show that the spectra are fit well with a combination of two
hydrogen model atmospheres, as long as the lines of the higher-gravity
component are broadened significantly relative to what is expected from just
pressure broadening. Interpreting this additional broadening as due to
rotation, the inferred spin period is short, about 1 minute. Similarly rapid
rotation is only seen in accreting white dwarfs that are magnetic; empirically,
it appears that in non-magnetized white dwarfs, accreted angular momentum is
lost by nova explosions before it can be transferred to the white dwarf. This
suggests that the massive white dwarf in SDSS 1257+5428 is magnetic as well,
with B~10^5 G. Alternatively, the broadening seen in the spectral lines could
be due to a stronger magnetic field, of ~10^6 G. The two models could be
distinguished by further observations.Comment: 9 pages, 3 figures, submitted to Ap
PTF 11kx: A Type Ia Supernova with a Symbiotic Nova Progenitor
There is a consensus that type Ia supernovae (SNe Ia) arise from the thermonuclear explosion of white dwarf stars that accrete matter from a binary companion. However, direct observation of SN Ia progenitors is lacking, and the precise nature of the binary companion remains uncertain. A temporal series of high-resolution optical spectra of the SN Ia PTF 11kx reveals a complex circumstellar environment that provides an unprecedentedly detailed view of the progenitor system. Multiple shells of circumstellar material are detected, and the SN ejecta are seen to interact with circumstellar material starting 59 days after the explosion. These features are best described by a symbiotic nova progenitor, similar to RS Ophiuchi
SN1998bw: The Case for a Relativistic Shock
SN1998bw shot to fame by claims of association with GRB980425. Independent of
its presumed association with a GRB, this SN is unusual in its radio
properties. A simple interpretation of the unusually bright radio emission
leads us to the conclusion that there are two shocks in this SN: a slow moving
shock containing most of the ejecta and a relativistic shock (Gamma=2) which is
responsible for the radio emission. This is the first evidence for the
existence of relativistic shocks in supernovae. It is quite plausible that this
shock may produce high energy emission (at early times and by inverse Compton
scattering). As with other supernovae, we expect radio emission at much later
times powered primarily by the slow moving ejecta. This expectation has
motivated us to continue monitoring this unusual SN.Comment: A&A (in press), Rome GRB Symposium, Nov. 199
Formation of rectifier with gold nanoclusters
Gold nanoclusters encapsulated with organic molecules are of great interest
for its possible applications in the fields of molecular electronics, catalysis
and medical science. Here we demonstrate that monolayer and bilayer films of
thiol-capped gold nanoclusters can exhibit diode-like properties provided
controlled spatial asymmetry exist between two tunnel junctions used to connect
a thiol capped gold nanoclusters. Current-voltage characteristics of this
rectifier were obtained from conducting probe atomic force microscopy
measurements and also from conventional two probe resistance measurements.
Systematic x-ray reflectivity and atomic force microscopy measurements were
carried out to characterize the spatial asymmetry introduced by a monolayer of
fatty acid salt gadolinium stearate used to deposit thiol-capped gold
nanocluster molecules on hydrophilic SiO2-Si(001) substrate by Langmuir
Blodgett technique. This information was used to explain prominent
rectification observed in these nano-structured films.Comment: 13 pages, 3 figure
A Luminous Companion to SGR 1806-20
We have obtained infrared spectra of the star suggested to be the counterpart
of the soft gamma-ray repeater (SGR) 1806-20. We found strong emission lines
similar to those seen in the spectra of the rare Luminous Blue Variables and
B[e] stars. A He I absorption line is also seen, from which we infer a spectral
type O9--B2. This classification, in combination with the minimum distance of
\simgt6 kpc inferred from its extinction, makes the star one of the most
luminous in the Galaxy. We infer that it is a companion to SGR 1806-20, and
suggest that the presence of a companion is somehow related to the SGR
phenomenon.Comment: 5 pages, AASTEX text+table and 2 PostScript figures (needs LaTeX
style files aaspptwo.sty, epsf.sty and rotate.sty). In case of problems,
contact [email protected]. Postscript file of complete article available
on request. (Replaced because first version had one wrong reference in it
PTF 10bzf (SN 2010ah): A Broad-Line Ic Supernova Discovered by the Palomar Transient Factory
We present the discovery and follow-up observations of a broad-line Type Ic supernova (SN), PTF 10bzf (SN 2010ah), detected by the Palomar Transient Factory (PTF) on 2010 February 23. The SN distance is â
218 Mpc, greater than GRB 980425/SN 1998bw and GRB 060218/SN 2006aj, but smaller than the other SNe firmly associated with gamma-ray bursts (GRBs). We conducted a multi-wavelength follow-up campaign with Palomar 48 inch, Palomar 60 inch, Gemini-N, Keck, Wise, Swift, the Allen Telescope Array, Combined Array for Research in Millimeter-wave Astronomy, Westerbork Synthesis Radio Telescope, and Expanded Very Large Array. Here we compare the properties of PTF 10bzf with those of SN 1998bw and other broad-line SNe. The optical luminosity and spectral properties of PTF 10bzf suggest that this SN is intermediate, in kinetic energy and amount of ^(56)Ni, between non-GRB-associated SNe like 2002ap or 1997ef, and GRB-associated SNe like 1998bw. No X-ray or radio counterpart to PTF 10bzf was detected. X-ray upper limits allow us to exclude the presence of an underlying X-ray afterglow as luminous as that of other SN-associated GRBs such as GRB 030329 or GRB 031203. Early-time radio upper limits do not show evidence for mildly relativistic ejecta. Late-time radio upper limits rule out the presence of an underlying off-axis GRB, with energy and wind density similar to the SN-associated GRB 030329 and GRB 031203. Finally, by performing a search for a GRB in the time window and at the position of PTF 10bzf, we find that no GRB in the interplanetary network catalog could be associated with this SN
A Radio Flare from GRB 020405: Evidence for a Uniform Medium Around a Massive Stellar Progenitor
We present radio observations of GRB 020405 starting 1.2 days after the
burst, which reveal a rapidly-fading ``radio flare''. Based on its temporal and
spectral properties, we interpret the radio flare as emission from the reverse
shock. This scenario rules out a circumburst medium with a radial density
profile \rho ~ r^{-2} expected around a mass-losing massive star, since in that
case the reverse shock emission decays on the timescale of the burst duration
t~100 s. Using published optical and X-ray data, along with the radio data
presented here, we further show that a self-consistent model requires
collimated ejecta with an opening angle of 6 degrees (t_j~0.95 days). As a
consequence of the early jet break, the late-time (t>10 days) emission measured
with the Hubble Space Telescope significantly deviates from an extrapolation of
the early, ground-based data. This, along with an unusually red spectrum, F_\nu
\~ \nu^{-3.9}, strengthens the case for a supernova that exploded at about the
same time as GRB 020405, thus pointing to a massive stellar progenitor for this
burst. This is the first clear association of a massive progenitor with a
uniform medium, indicating that a \rho ~ r^{-2} profile is not a required
signature, and in fact may not be present on the lengthscales probed by the
afterglow in the majority of bursts.Comment: Submitted to ApJ; 14 pages, 2 tables, 3 figure
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